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present experimental conditions. The possible reasons for the nucleate pool boiling heat transfer enhancement are as follows. (1) The heating surface characteristics are changed due to the interaction between nanoparticles and heating surface. According to the principle proposed by Prakash et al. [25], when the ratio of heating surface roughness to nanoparticle diameter (Ra/ dp) is much larger than 1, the number of nucleation sites greatly in- crease. Ra/dp in the present study is 80, so the interaction between nanoparticles and heating surface causes the increase of the active nucleation site density, which leads to the enhancement of the nucleate pool boiling heat transfer. (2) The thermophysical properties are changed due to the pres- ence of nanoparticles. The thermal conductivity of Cu nanoparticle (398 W m1K1) is four orders of magnitude higher than that of R113 (0.064 W m1K1), so the thermal conductivity of Cu-R113 nano- fluid is larger than that of pure R113, causing the enhancement of micro-layer evaporation [26], which leads to the enhancement of the nucleate pool boiling heat transfer. In order to analyze the effect of nanaoparticles on the nucleate pool boiling heat transfer coefficient quantitatively, nanoparticle enhancement ratio, NER, is defined in this paper, as shown in Eq. (5): NER 1⁄4 hr;n =hr ð5Þ where hr,n and hr are the nucleate pool boiling heat transfer coeffi- cient of refrigerant-based nanofluid and that of pure refrigerant, respectively. Fig. 4 shows the nanoparticle enhancement ratio (NER) at nanoparticle concentrations of 0.1 wt.%, 0.5 wt.% and 1.0 wt.%, respectively. From Fig. 4, it can be seen that NER is in range of 1.30–1.55, and increases with the increase of nanoparticle concentration. The possible reasons for this phenomenon are as follows. (1) The interaction between nanoparticles and heating surface increases with the increase of nanoparticle concentration, causing the larger increase of the active nucleation site density, which leads to NER increasing with the increase of nanoparticle concentration. (2) The thermal conductivity of Cu-R113 nanofluid increases with the increase of nanoparticle concentration, causing the larger enhancement of micro-layer evaporation, which leads to NER increasing with the increase of nanoparticle concentration. When comparing the present experimental data of Cu-R113 nanofluid with the experimental data of TiO2–water nanofluid presented by Suriyawong and Wongwises [27] as well as the experimental data of TiO2-R141b nanofluid presented by Trisaksri and Wongwises [18], it can be found that the nanoparticle type and concentration, the surface roughness and material, and the host fluid type have effects on the nucleate pool boiling heat transfer. H. Peng et al. / Experimental Thermal and Fluid Science 35 (2011) 960–970 965 Fig. 6. Nucleate pool boiling heat transfer of Cu-R113 nanofluid with anionic surfactant (SDS). (a) Boiling curves (heat flux vs. excess temperature) (b) Heat transfer coefficients vs. heat fluxPDF Image | Experimental Thermal and Fluid Science 35
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